1. Field of the Invention
This invention relates to activated carbon used for vapor phase operations, especially solvent recovery operations. More specifically, this invention relates to granular activated carbon that has been modified to minimize or prevent the occurrence of granular activated carbon bed fires during and after adsorption of volatile organic compounds from gas streams.
2. Description of the Prior Art
Granular activated carbon (GAC) has been a technically and economically successful material for use in solvent recovery from gas streams for many years. Solvent recovery by GAC adsorption is usually a batch operation involving multiple beds. At least one GAC bed remains online while others are being regenerated. The adsorber inlet gas stream is typically pre-treated to remove solids (dust), liquids (droplets or aerosols), or high-boiling components that may hamper the performance of the GAC bed. The filtered solvent-laden inlet gas stream is passed upwardly through the fixed GAC bed, which adsorbs the solvent from the solvent-laden gas stream.
GAC that has adsorbed its capacity of solvent or chemical contaminants can be reused if the adsorbed solvent is removed. This is termed regeneration of the GAC. This regeneration can be done either by vapor stripping with steam or hot nitrogen, or by removing the carbon from the bed and thermally reactivating it on site or off site. Spent GAC is usually regenerated with downward flowing low-pressure steam or hot nitrogen gas. This removes the adsorbed solvent, which is typically recovered by condensing the vapors and separating the solvent from water by either decantation or distillation. After steam regeneration, the hot wet GAC bed is dried, typically by use of a hot air stream. The GAC bed is then cooled by an ambient temperature air stream prior to beginning its next adsorption cycle. These methods leave a heel of material in the carbon pores that eventually necessitate the thermal reactivation of the carbon, either on site or off site.
By design, GAC systems can adsorb up to 30-40 wt. % organics. Unfortunately, carbon-based adsorption systems can also sustain oxidation at fairly low even ambient temperatures. This is particularly true if ketones and aldehydes are part of the solvent mixture being adsorbed. Methyl ethyl ketone (MEK) and cyclohexanone (CH) are particularly prone to oxidation. The GAC can act as a catalyst for this oxidation (Henning et al., xe2x80x9cPurification of Air, Water and Off Gas Solvent Recoveryxe2x80x9d, 19th Biennial Conference on Carbon, Jun. 25-30, 1989, pp. 1-10). The oxidation will often ultimately result in spontaneous combustion in GAC beds, either in service or idle.
GAC bed fires occur when the heat gain from the heat of adsorbtion of organics and oxidation of the adsorbed organics outpaces any heat loss via conductive or convective cooling mechanisms. Such conditions can lead to a thermal runaway.
One method that has been used to attempt to minimize the occurrence of GAC bed fires is to employ engineering design controls (Hofelich et al., xe2x80x9cPrevent Thermal Runaways in Carbon Bedsxe2x80x9d, Chemical Engineering, January 2000, pp. 99-102). These methods entail monitoring the GAC bed temperature for indications of oxidation occurring. If the GAC bed temperatures indicate a potential runaway situation, controls such as flooding the GAC bed with inert gas are initiated to stop the reaction. These engineering solutions have proven to be marginally successful, as localized xe2x80x9chot spotsxe2x80x9d in the GAC bed are not always detected. This can result in a GAC bed fire beginning before the engineering controls detect that a problem exists.
U.S. Pat. No. 5,073,454 to Graham discloses coating activated carbon with an intumescent material, which will expand at an elevated temperature and render the activated carbon resistant to oxidation. This method, however, decreases the adsorption capacity of the activated carbon, especially at elevated temperatures, all the way to completely removing the adsorptive capacity.
U.S. Pat. No. 5,217,505 to Maroldo et al. discloses a process for removing an oxidizable organic compound from an oxidizing gas stream which includes contacting the gas stream with a GAC bed which also includes porous pyrolyzed particles of a polysulfonated cross-linked synthetic copolymer. The pyrolyzed copolymer is added to adsorb the oxidizable organic compound. This method decreases the amount of oxidizable organic compounds adsorbed on GAC, but does not eliminate it. Thermal excursions and GAC bed fires are still able to occur using the above-described method.
There remains an established need to prevent the occurrence of GAC bed fires in such a way as to not decrease the adsorption potential of the GAC. In order to be economical, any method for preventing GAC bed fires must also provide for the regeneration of the GAC for reuse.
The present invention is directed to an activated carbon composition that is resistant to oxidation and the activated carbon bed fires that can result from uncontrolled oxidation of the activated carbon bed. The activated carbon composition of the present invention includes from about 0.0001-25 wt. % of an antioxidant and from about 75-99.9999 wt. % activated carbon, including any and all other impregnants and moisture. The presence of the antioxidant on the surface of the activated carbon inhibits uncontrolled oxidation and the activated carbon bed fires that can result.
The present invention is also directed to a method of preventing activated carbon bed fires. The method includes treating the activated carbon to be used in the bed with an antioxidant such that the resulting activated carbon composition includes from about 0.0001-25 wt. % of an antioxidant and from about 75-99.9999 wt. % activated carbon including any and all other impregnants and moisture.
The present invention also provides a method of removing solvents from a solvent laden gas. The method includes passing the solvent laden gas through an activated carbon bed which contains an activated carbon composition that includes from about 0.0001-25 wt. % of an antioxidant and from about 75-99.9999 wt. % activated carbon including any and all other impregnants and moisture.
The present invention is further directed to a method of regenerating an oxidation resistant activated carbon that has been saturated with organic material. The method includes the steps of heating the activated carbon to a temperature in excess of about 200xc2x0 F., by applying downward flowing low-pressure steam or hot nitrogen to remove the organic material from the surface of the activated carbon, and treating the activated carbon with an antioxidant such that the resulting activated carbon composition includes from about 0.0001-25 wt. % of an antioxidant and from about 75-99.9999 wt. % activated carbon including any and all impregnants and moisture.
The present invention is also directed to a method of thermally regenerating an oxidation resistant activated carbon that has been saturated with organic material. The method includes the steps of placing the activated carbon in a direct or indirect fired kiln or Hirschoff furnace; heating the activated carbon to about 1,000-2,000xc2x0 F. in an activating gas atmosphere to remove the organic material from the surface of the activated carbon maintaining the kiln or furnace temperature for 1-12 hours and cooling the activated carbon to ambient temperature; and treating the activated carbon with an antioxidant such that the resulting activated carbon composition includes from about 0.001-25 wt. % of an antioxidant and from about 75-99.9999 wt % activated carbon including any and all other impregnants and moisture.